1. Field of the Invention
This invention relates to systems for spacing surfaces of potentially varying temperature, and specifically for rigidly spacing the walls of nested vessels in a cryostat when the vessels are at substantially the same temperature and eliminating heat conduction paths between such vessels through the spacing system when low temperature liquified gases are retained in the cryostat.
2. Description of the Prior Art
Cryostats are often used for the containment of superconducting apparatus such as superconducting magnets. A magnet coil is maintained at very low temperature by an envelope of liquid helium. The liquid helium is further surrounded by various insulating envelopes from the ambient temperature, including typically a surrounding layer of liquid nitrogen.
Cryostats have typically taken the form of nested vessels which are internally braced to maintain minimum clearances between adjacent nested vessel walls. It is often desireable to assemble a cryostat prior to shipment to its ultimate working location. Thus, the internal bracing must be rigid enough to withstand the mechanical loads (in all directions) which can occur during the shipping process. This has resulted in rather complicated spacing and bracing schemes. Stainless steel spokes have been used in order to withstand mechanical shock between walls. Such spokes, however, provide heat conduction paths between the walls of the vessels, which are to be maintained at different temperatures when the cryostat is in use in order to create the various thermal insulation envelopes.
One attempt to eliminate such heat conduction is shown in U.S. Pat. No. 4,212,169, granted to Kneip, Jr. on July 15, 1980 and which is hereby incorporated by reference. The vessels of the Kneip, Jr. cryostat are spaced apart by a plurality of polyester cord fasteners. While the use of this material does reduce heat conduction, there is still a direct material path between adjacent vessel walls through such cord fasteners when low temperature liquified gases are retained in the cryostat.
Other attempts to thermally insulate liquified gas containers are shown in U.S. Pat. No. 4,038,832, granted to Lutgen et al. on Aug. 2, 1977 and U.S. Pat. No. 3,839,981, granted to Gilles on Oct. 8, 1974 which are hereby incorporated by reference. These patents show studs extending from outer walls of an inner vessel to be retained within a bracket mounted on a next vessel or framework. When liquified gas is introduced into the inner vessel, contraction of the studs with respect to the brackets occurs, but there is still contact between the studs and brackets and thus the heat conduction path between the inner and outer vessels is never completely broken. U.S. Pat. No. 2,911,125, granted to Dosker on Nov. 3, 1959 and incorporated by reference herein, shows a storage tank for cold liquids which is supported with respect to an outer frame during contraction by a plurality of annular rings received within grooves in the tank. Upon the introduction of cold liquids into the tank, the tank contracts, but the rings are never withdrawn from the grooves, thereby continually providing heat conduction paths between the framework and the tank. U.S. Pat. No. 3,007,598, granted to Beam on Nov. 7, 1961 and U.S. Pat. No. 2,954,003, granted to Farrell et al. on Sept. 27, 1960 which are hereby incorporated by reference, both show means for transportation of low temperature liquids in tanks. The tanks are supported with protrusions therefrom fitted in recesses in a next outer tank. The patents show and discuss these protrusion and recess arrangements for nested containers and the relative movements thereof, but there is no anticipation of the complete elimination of contact between the innerlocking parts of the containers of these patents.
Prior art spacing systems for nested temperature vessels such as those discussed above have been unsuitable in practice for a variety of reasons. When the spacing system comprises rigid members such as stainless steel spokes, the spokes permit heat transfer between temperature vessels. The use of polyester cords reduces, but does not eliminate such heat conduction and provide no rigidity between vessels during transport of the cryostat. The other systems shown and discussed above also do not eliminate direct heat conduction paths between adjacent nested vessel walls when low temperature liquified gases are stored therein.